There are numerous methods for automatically focusing a camera. Many of these methods have a common basis in that the high frequency components of the image are detected and accumulated for various focal positions of the lens. By comparing the present value of the accumulated high frequency components with their previous accumulated value, it can be determined at what lens position the high frequency components are at a maximum. This lens position is concluded to be the optimum focal position.
Somewhat related autofocus systems are discussed in U.S. Pat. No. 3,713,371 by Kurihara et al, where there is disclosed an autofocus system which operates as a function of the difference in brightness levels between adjacent elements of an image.
In U.S. Pat. No. 4,392,726 by Kimura there is disclosed an automatic focus control system which determines an optimum focus condition to exist when the high frequency components of the image are at a maximum value.
Furthermore, Fukuoka in U.S. Pat. No. 4,183,642, discloses an autofocus device which involves calculating a difference in brightness information between adjacent picture elements, effecting an operation to provide smoothness of the brightness change associated with this difference, providing an accumulated value of the smoothness of the brightness change with respect to all picture elements, and adjusting the focus to maximize this accumulated value.
And, in U.S. Pat. No. 4,437,743 by Sakai et al there is disclosed a signal processing system for processing signals from accumulating type photoelectric transducer elements wherein a means is incorporated for varying a threshold level of an input signal.
Due to the presence of noise, however, the method of adjusting the lens position until the high frequency components are maximized sometimes results in less than optimum focus. That is, the presence of high frequency noise can result in a maximum signal when the lens is still out of focus. It is therefore desirable to provide an autofocus system which displays some immunity to noise to generate a control signal for accurately achieving autofocus.
In addition, where the high frequency components of the video signal are accumulated to detect a maximum, the hardware necessary to generate this large accumulated value can be substantial. It is desirable therefore to reduce the hardware requirement while still generating accurate autofocus information.